Method of increasing sinter rate



Oct. 16, 1956 c. BRADWELL 2,767,074

METHOD OF INCREASING SINTER RATE Filed June 22. 1951 2 Sheets-Sheet 1Stem For K117019171?! J i HINVENTOR Cyril firWelL.

856m jz BY g I I ATTO EY METHOD OF INCREASING SINTER RATE CyrilBradwell, Bethlehem, Pa., assignor to Bethlehem Steel Company, acorporation of Pennsylvania Application June 22, 1951, Serial No.232,997

Claims. (Cl. 75-59 My invention relates to a method of increasing thespeed of a sintering operation by means of preheating the mix.

In conventional sintering operations With iron ores, flue dust,concentrates and other materials, preparatory to charging into the blastfurnace, the ore or the like is mixed with a finely divided fuel such ascoal, coke, flue dust, sawdust, etc., and with some return sinter fines,to which a rather critical amount of cold water for tempering is added,and the moistened mix is then sintered on a conventional sinteringmachine. The operation of the sintering machine is such that the mix isdeposited on a grate above wind boxes connected to a powerful suctionfan or fans and its upper surface is traversed by a flame from asuitable ignition burner. Since sintering rate for a given mix increaseswith increase in air flow through the bed, for maximum speed ofsintering, it is obvious that the bed must remain as permeable aspossible after ignition has taken place.

As delivered to the sintering machine, a great deal of attention isdirected to maintaining the mix at the optimum Water content value,roughly about 6% to 14%, depending on the mix, which has been found toproduce optimum permeability. Mixes which are too wet will not sintereffectively, and a mix which is excessively dry will have greatlyreduced permeability and also too many of its particles will be pulledthrough the grate bars by the fan suction. The distribution of thiswater content, moreover, is intentionally made very uniform throughoutthe mix prior to depositing it on the machine.

This carefully predetermined moisture distribution often is upset,however, under the combined action of the burner and the downdraft, assoon as ignition and sintering begin. Water from the top portions of thebed is vaporized and carried downward by the combustion gases and freeair to the cold lower portions of the bed where, frequently, some of itscondenses. This action destroys much of the original permeability of themix which was carefully developed by proper mixing and by close controlof the amount of water added. With the flow of air through the bed thusreduced, the speed at which sintering can proceed is also reduced.

The object of this invention, therefore, is to prevent condensation ofmoisture in the bed and thereby to assist in maintaining thepermeability of the bed to the flow of air during sintering.

I have discovered that by preheating the wet mix prior to sintering Ican eliminate this condensing elfect, and thereby increase the sinterrate at a rate increasing with increasing temperature of the raw mix.

The unsintered mix, of course, may more or less accidentally contain acertain amount of residual heat, since the solids in a typical batch mayinclude about to 40% return sinter fines together with the ores,concentrates and/ or flue dust. The ores, concentrates and flue dust arealways used cold, at approximately room temperature. If return sinterfines are used, these may be hot if returned immediately, butoccasionally they are nited States Patent 0 2,767,074 P atented Oct. 16,1956 stored in bins so that the percentage added can be controlled andwhen this is done the returns will lose much or all of their heat.Practically speaking, therefore, the amount of heat obtained from thefines is unpredictable, because the temperature and quantity of thereturns are constantly changing and the differences in specific heats ofthe various materials introduce other variant factors. In any event, theamount of heat obtainable from return sinter fines is insuflicient forthe purposes of this invention.

Even after adding hot return fines, much of the heat from the returnfines will be dissipated before the mix reaches the sintering machine,while it travels exposed to the air on conveyor belts and falling freelythrough open hoppers. Moreover, the addition of cold water for temperingwill further reduce the temperature of the mix.

Since preheating is desired, it might seem convenient to raise thetemperature of the mix simply by increasing the percentage of returnedhot sinter, but as an economic matter this would merely increase therecirculating load with a concomitant waste of good blast furnace feed,and most of the heat would still be lost as before.

Because heat is lost by the mix in its travel through the plant, it isdesirable to add heat to the mix as near as possible to the machine.

One means of increasing the mix temperature is to use hot water to bringthe mix to its optimum moisture content rather than cold water as is thepresent custom. Because the amount of water which can be added isrestricted by the considerations mentioned heretofore, hot water yieldsonly a partial solution to the problem.

I therefore prefer to obtain the desired additional heat by the use ofsteam in the hoppers, bins, conveyor belts and, if applicable, in thefluffer mixer, pugmill or other mixing apparatus just before themachine. One method of preheating the mix by means of steam is shown inthe drawings.

In the attached two sheets of drawings:

Fig. 1 is a diagram illustrating a method of preheating the mix by meansof steam, as applied to conventional sintering plant apparatus;

Fig. 2 is a detail view of a jet arrangement for admitting steam tohopper;

Fig. 3 is a detail view of a steam jet arrangement for conveyor belt;and

Fig. 4 is a graph with curves indicating the general effect ofincreasing preheating temperatures upon sinter mixes of differentoriginal temperatures and water contents.

With reference to the drawings, and more particularly to Fig. 1, thenumeral 1 designates a conveyor belt or so-called J-belt, from which themix 2 falls through J-hopper 3 onto covered K-belt 4, upon which the mixis advanced and drops through K-hopper 5 into a rotating drum typeflutfer mixer 6 or the like. From the fluifer mixer 6 the mix dropsthrough hopper 7, swinging spout 8 and hopper 9 onto the moving grate 10of a sintering machine, on which the mix travels under ignition arch 12and over wind boxes 13 and is ignited and sintered. By means ofmulti-jet perforated pipes 14 (Fig. 3), 15 (Fig. 2) and 15, steam isinjected into the mix on the K-belt 4, in the K-hopper 5, and fluffermixer 6, respectively. In similar manner, hot water in suitable quantityfor tempering is added by means of perforated pipe 17 in flutter mixer6.

There are obviously too many variables, such as differences in moisture,carbon, mix composition, machine speed, and charging conditions, all ofwhich may affect sinter rate, for it to be possible to state withexactness the percentage of gain which may be expected at any particularpreheat temperature. Merely as an illustration, however, a typicalsinter mix with 9% water content preheated to F. was found to produce38% more sinter by weight in a given time than the same mix sinteredunder the same conditions but preheated only to 120 F. Other results areshown on the attached curves (Fig. 4).

'The curves in Fig. 4 illustrate the effect of preheating one typicalmix whose optimum moisture content is 9% when it is prepared andsintered in the normal manner. The solid line shows how the weight ofsinter produced in a given time increases with increase in preheattemperature of the mix.

When this same mix was prepared with 10% moisture and sintered withoutpreheating, its rate of sintering decreased as shown by the dashed curvebelow 115 F. However, by preheating the mix to temperatures above 115F., its sintering rate Was greatly increased The increases, due topreheating, shown on both curves occur not only because condensation isprevented but also because the gases moving through the preheated bedastually partially dry the lower layers and thus aid combustion of thefuel.

In this particular mix, it should be noted, 8% water was not sufficientto develop maximum original permeability.

In the sintering operation a certain strength of product is desired andthis strength is largely controlled in a given mix by the amount ofcarbon or fuel introduced. Within limits, the sinter strength increaseswith increase in carbon content of the mix. During sintering most of theheat developed by the burning fuel wholly or partially fuses thematerials and develops strength in the product but some of the heatreleased by the burning fuel is consumed in evaporating moisture. Bypreheating, condensation of moisture in the bed is prevented andtherefore more of the heat developed by the burning fuel is available tofuse the mass. It therefore follows that if the mix is preheated, lesscarbon will be needed to develop a given strength of product. Since thesintering rate of a mix decreases as fuel content increases (withinlimits), a further gain in sintering rate is obtained by preheatingbecause the fuel content of the mix can be decreased and still produce asinter of equal strength to that obtained from the same mix sinteredwithout preheating.

I have further found that many extremely fine grained materials such asfor instance taconite concentrates, usually regarded as very slow orahnost impossible to sinter without special treatment such as nodulizingand/or mixing with other ores or return fines, may be sintered at veryrapid rates provided only that they are mixed correctly with fuel andwater and preheated before they are fed to the sintering machine.

The preheating of the mix to prevent condensation of moisture from theair and combustion gases will also be beneficial in connection with thesintering of any of the non-ferrous sinterable materials well-known inthe art Where such condensation decreases the flow of air through thebed.

Although I have thus described my invention in considerable detail, I donot wish to be limited narrowly to the exact and specific preheatingmeans and temperatures mentioned hereinabove, but I may also use suchsubstitutes, modifications, or equivalents thereof as are embracedwithin the scope and spirit of the invention and of the appended claims.Nor do I wish to be limited to the type of sintering machine describedherein, since the advantages of the process will apply to any sinteringapparatus.

I claim:

1. The method of sintering which comprises delivering to the grate of asintering machine a permeable mixture of finely divided sinterablematerial, fuel and water at a temperature in the range between F. and F.and rapidly sintering the mixture without substantial condensation ofvapor in said mixture.

2. The method of sintering which comprises delivering to the grate of asintering machine a permeable mixture of finely divided iron ore andfuel having a moisture content in the range between 6% and 14% andhaving a temperature in the range between 120 F. and 195 F. andsintering the said mixture very rapidly and without substantialcondensation of vapor.

3. The method of sintering a mixture of small particles of iron ore,fuel and water which comprises agitating the mixture while injectingsteam therein to produce a mixture having a water content in the rangebetween 6% and 14% and having a temperature in the range between 120 F.and 195 F., delivering the heated mixture to the grate of a sinteringmachine, and sintering the said mixture very rapidly and substantiallywithout condensation of vapor in the lower layers of the mixture.

4. The method of sintering a mixture of finely divided sinterablematerial, fuel and water which comprises heating the mixture to atemperature in the range between 120 F. and 195 F., delivering theheated mixture of sinterable material, fuel and water to the grate of asintering machine, and sintering the said mixture in the absence ofmoisture of condensation from hot gases.

5. The method of sintering a mixture of sinterable fine material, fueland water which comprises heating the mixture by means of steam to atemperature in the range between 120 F. and 195 F, delivering the heatedmixture of sinterable material, fuel and Water to the grate of asintering machine, and igniting and sintering the said mixture Withoutsubstantial condensation of vapor from hot gases in their passagetherethough.

References Cited in the file of this patent UNITED STATES PATENTS852,611 Perkins et a1. May 7, 1907 1,632,829 Fleissner June 21, 19272,052,329 Wendeborn Aug. 25, 1936 2,119,615 Wendeborn June 7, 19382,532,335 Royster Dec. 5, 1950 2,544,742 Gelbman Mar. 13, 1951 2,608,481Royster Aug. 26, 1952 FOREIGN PATENTS 229,608 Great Britain Feb. 26,1925 573,539 Great Britain Nov. 26, 1945 OTHER REFERENCES Metallurgy ofCopper, by Newton and Wilson. Pub. 1942 by John Wiley and Sons, Inc.,New York. Pages 59 and 60.

Websters New International Dictionary of the English Language, 2nd ed.,unabridged, pub. 1940 by G. and C. Merriam Co., Publishers, Springfield,Mass. Page 2346.

1. THE METHOD OF SINTERING WHICH COMPRISES DELIVERING TO THE GRATE OF ASINTERING MACHINE A PERMEABLE MIXTURE OF FINELY DIVIDED SINTERABLEMATERIAL, FUEL AND WATER AT A TEMPERATURE IN THE RANGE BETWEEN 120* F.AND 195* F. AND RAPIDLY SINTERING THE MIXTURE WITHOUT SUBSTANTIALCONDENSATION OF VAPOR IN SAID MIXTURE.